Gas supplying a property may carry contaminant particles. It is important that gas meters, in particular ultrasonic gas meters, through which gas enters a property are protected from such particles in order to ensure proper working of the meter. Therefore it is necessary that any contaminant particles that are present in the gas stream are removed, at least to an acceptable level, prior to entry of the gas to a measurement unit of the meter.
Techniques of passing the gas through a filter prior to the gas entering the measurement unit of the meter are known. However, this approach has the drawback that, with time, the filter becomes clogged by the contaminant particles, thereby obstructing the gas flow and causing the gas pressure entering the meter to exceed a permissible level.
In accordance with European standard EN 14236, which is herein incorporated by reference, ultrasonic gas meters must be tested to ensure that they are immune to four separate batches of dust, each batch having 95% of the appropriate size range given below:                a) 0 μm to 100 μm Average size (50±10) μm;        b) 100 μm to 200 μm Average size (150±10) μm;        c) 200 μm to 300 μm Average size (250±10) μm;        d) 300 μm to 400 μm Average size (350±10) μm.        
Each of the above batches shall have a composition by mass of:                Black iron oxide (Fe3O4) 79%        Red iron oxide (FeO) 12%        Mineral silica flour (SiO) 8%        Paint residual flake 1%        
To pass the test, a gas meter filter must not become clogged by the largest particles used in the test.